1. Field of the Invention
This invention relates to a dynamic pressure utilizing gas bearing device whereby the shaft rotating at high speed is rotatably supported in a levitated state by the action of a gas film or layer.
2. Description of the Prior Art
The conventional dynamic pressure type gas bearing devices had problems in that excess pressure would be loaded to the bearing pads or the shaft and bearing pads would be overheated to cause seizure when the gas layer thickness is decreased with thermal expansion of the shaft. The present invention is envisaged to overcome such problems by the combined use of spring and gas supports.
A typical example of the conventional dynamic pressure type gas bearing mechanism is shown in FIG. 1. The bearing mechanism shown in FIG. 1 includes three plate-shaped bearing pads 2a, 2b and 2c arranged equidistantly from each other around a shaft 1. These bearing pads have a slightly larger radius of curvature than the shaft 1 and all have the same thickness, with their outer peripheral faces being supported by balls 3a, 3b, 3c carried at the ends of pivots 7a, 7b, 7c, respectively. The pivots 7b and 7c are each screwed into a bearing support block 5 and, after positional adjustment, locked in position by a lock nut 8. The pivot 7a supporting the bearing pad 2a positioned upwardly of the shaft 1 is so adjusted positionally as to provide a predetermined amount of clearance a between the shaft 1 and the bearing pad 2a, and then is fixed to a fixing portion 4a at one end of a leaf spring 4 by means of a lock nut 8. Leaf spring 4 is secured at its other fixing portion 4b to a bearing support 5 by a bolt 6.
In FIG. 1, when the shaft 1 is rotated in the direction of arrow A, the viscosity of the ambient gas causes it to be forced between the bearing pads 2a, 2b, 2c and the external surface of the shaft 1; consequently, a gaseous film or layer having a higher pressure than the ambient gas is formed between the external surface of the shaft 1 and the internal faces of the bearing pads 2a, 2b, 2c by the wedging action of the sucked-in gas. As three or more bearing pads are arranged equidistantly from each other around the outer periphery of the shaft 1, the shaft 1, when rotated at a high speed, is lifted up by the action of combined pressure of the gaseous film formed between the inner surfaces of the respective bearing pads 2a, 2b, 2c and the opposed outer surface of the shaft 1. At this time, the center of rotation of the shaft 1 is located at a position slightly lower (by a distance equal to the amount of descent of the shaft due to its own weight) than the geometrical center of the bearing pads. It is also known that the smaller the value of clearance a, the greater the pressure of the gaseous film.
However, such conventional dynamic pressure type gas bearing mechanisms present various problems, such as mentioned below, when high-frequency vibration is imparted thereto due to exposure to external vibratory forces or unbalance remaining in the revolving shaft 1 and other rotating members mounted on the shaft 1. For instance, when dynamic pressure type gas bearing unit of the above-described type is used in a high heat generating engine, such as a gas turbine engine, the shaft 1 could be very quickly heated due to quick variation of load. At such a time, if the outer diameter of the shaft 1 is expanded by the elevated heat, the clearance a is correspondingly reduced to cause a rise of pressure of the gaseous film between the shaft 1 and bearing pads 2a, 2b, 2c, with the result that pads 2a, 2b, 2c are forced outwardly by such elevated pressure of the gaseous film. The bearing pads 2b and 2c cannot move in the outward direction as they are held by the pivots 7b and 7c secured to their respective bearing support blocks 5. But, the bearing pad 2a positioned upwardly of the shaft 1 is permitted to move outwardly surmounting the resisting force of the leaf spring 4 as the external face of pad 2a is supported by a pivot 7a which is secured to leaf spring 4. However, the leaf spring 4 has a large spring constant so as to inhibit the shaft 1 from moving to any excess degree when a sharp impact force is exerted on the shaft 1. Therefore any rise in pressure of the gaseous film cannot produce any appreciable degree of displacement of the leaf spring or bearing pad 2a, so that the clearance a remains small. Consequently, as the pressure of the gaseous film is raised, the bearing pads receive an excessive pressing force from the respective pivots 7a, 7b, 7c. Further, if thermal expansion of the shaft is excessively large, the gaseous film may become unable to withhold the pressing force of the leaf spring 4, causing seizure of the shaft and bearing pads. Also, when high frequency vibration is imparted to the bearing unit owing to an application of an external vibratory force or to unbalance remaining in the shaft and other rotating members mounted on said shaft, although the bearing unit still possesses a relatively high shaft supporting force, no satisfactory vibration attenuating effect is provided because internal friction of the leaf spring 4 is the only element that can act to attenuate vibration.